I don't know about you, but I know I've spent time on games where I went specifically after one achievement only to find out that I wasted a lot of time because I could have got it just playing the game normally. Well, this is one of those games and achievements.
While there is nothing wrong with farming coins for this achievement and others, there's really no point in this game if you are going for all of the other achievements as well.
Just natural playing through the Survival modes (which you will have to do to get Endless Survival for that achievement) will net you a LOT of plants for your Zen garden, which you can get $10k-$12k+ for (after the coins/gems you get before finally selling them).
To put this into perspective, I just finished the Endless Survival achievement, have already built the tree up 100 ft., have all of the flower upgrades from Crazy Dave, have a Night Garden, all of the unique Zen Garden items (glove, snail, etc.), a Level 5 Clown House, 18 plants in my Zen Garden, AND $114,000 still in the bank.
As you can see, I spent a lot of unnecessary money along the way and STILL have more than I would even need for anything else in the game.
Hope this helps save you some time!:)